1. Field of the Invention
The present invention relates to a method and apparatus for adjusting an exposure device suited for an electrophotographic printer. The present invention also relates to a an exposure device and a printing apparatus that includes the exposure device.
2. Description of Background Art
A category of non-impact printers makes use of an exposure device such as a printhead. A plurality of light-emitting elements record latent images on a photosensitive an exposure device may be provided with an array of light-emitting elements such as light emitting diodes (LEDs). A lens mechanism such as a rod lens array (commercially available under the trade-marked name SELFOC) can be used in the printhead for focussing the light emitted by the LEDs on the photosensitive recording member. Printers of the above mentioned type also include a developer that develops the latent image formed on the photosensitive member into a visual toner powder image. Such printers further include a transfer mechanism that transfers the toner powder image from the photosensitive recording member onto an image receiving medium such as a sheet of paper.
In exposure devices of the above mentioned type, the LEDs are mounted on a solid substrate and generally arranged in rows across the width of the photosensitive recording member. LED chips may be provided, each one of the chips containing for example a block of 128 integrated LEDs. A number of LED chips can be mounted on a module plate and several module plates can be mounted such that a print bar of a desired width is formed whereon LEDs are spaced with a constant pitch.
Energy output levels are applied to the LEDs by associated drivers, in order to produce light spots on the photosensitive receiving member for producing an image made of picture elements (pixels). Spots having multiple energy levels are obtained by providing multiple levels of output power for a constant period of time, or by providing a constant output power level for a period of time proportional to the gradation value of a pixel. In so-called binary printers, only two possible energy levels can be applied to an LED, one level for giving rise to a light spot, the other level being a zero energy level. If a charge area development process is used, a light spot projected on the photosensitive member with a light intensity larger than a so-called print threshold intensity is discharging locally the photosensitive material and no toner is developed locally (no pixel). If a charged area development is used and an LED is not driven (zero-energy level), the photosensitive member remains locally charged and toner is locally transferred for giving rise to a pixel. Although the present invention is described for a charged area development type of process, the present invention is also suitable for an uncharged area development type of process, making the required changes.
The unevenness of the optical density in printed images obtained with printers using such an exposure device that includes LEDs has to be minimized. Unevenness of the optical density in printed images may be caused by a large spread of the light intensities emitted by the LEDs due to a production process or material, temperature dependence of the LED output yield and differing light transparency of the lens mechanism (for example, a Selfoc lens array) across the print width. Another source for the unevenness of the optical density in printed images are local imperfections of the rod lens array, such as anomalous lens rod fibers or misaligned lens rod fibers. Unevenness of the optical density in printed images can also be caused by height differences of LEDs, or of LED-chips or of chip module plates. In order to minimize the unevenness of the optical density in printed images, setting values for the energy output level for driving each light-emitting element are determined, before the exposure device is mounted in the printing apparatus.
A method of the above type is known from U.S. Pat. No. 5,774,165. With the known method, although the light intensity distribution of each LED has substantially the same predetermined width at a predetermined light emission intensity, printed images still present unevenness of the printed optical density.